JSFiddle - React, Tailwind, and code Playground

JavaScript

function converterQS(str) {
    var data = str.split('?')[1];
    var pares = data.split('&');
    var obj = {};
    pares.forEach(function (par) {
        var partes = par.split('=');
        obj[partes[0]] = partes[1];
    });
    return obj;
}

console.log(converterQS('smartphones?marca=3')); // Object {marca: "3"}